A rev limiter is a device fitted to an internal combustion engine to restrict its maximum rotational speed. Rev limiters are pre-programmed into the car's Engine Control Unit, except aftermarket units, which are packaged as a separate micro-controller. Cars need a limiter because it prevents the engine from operating above a pre-determined RPM level known as the redline. If an engine goes over the redline, commonly called "Over-Revving", damage to the engine and valve train may occur because it can cause the valve to stay open longer than usual. Valve float can possibly result in loss of compression, misfire, or a valve and piston contacting each other.
Limiters usually work by cutting off a necessary component needed for the combustion processes, although some will change ignition timing. Commonly, rev limiters control the spark or the fuel injectors to the engine.
Types of Control
Ignition Control rev limiting systems work by interrupting the power that is distributed to the spark plugs. Once the car reaches the redline, the engine stops the spark. This is not very common in production cars because it is bad for emissions, as the system still injects fuel into the cylinder and consequently releases unburnt fuel out the exhaust pipe. This type of limiting can also affect the temperatures in the exhaust, causing premature wear on the catalytic converter.
Fuel-cutting rev limiters are the most common because they are better for emissions, and wear less on engine components. These systems usually start to lean out the engine at redline by cutting fuel to the injectors.
Hard Cut vs. Soft Cut
Hard-Cut limiters are a type of rev limiter that completely cuts fuel or spark to the engine. These types of limiters activate at the set RPM, usually redline, and "bounce" off of it if throttle is applied. The "bouncing" occurs because the limiter will cut off fuel or spark at the set RPM, which causes the RPM to drop. If the engine is in a state of open throttle when the RPM drops, the RPM will then raise back to the limit. This causes the engine to cycle between the RPM limit and just below.
Soft-Cut limiters are a type of rev limiter that partially cuts off fuel to the engine. These limiters may also retard the ignition timing. If using a soft-cut rev limiter, the engine will start to cut fuel or retard ignition timing before the set RPM until it slowly reaches it and remains there.
The maximum RPM of an engine is limited to the airflow into the engine, the displacement of the engine, the mass of the rotating parts, along with the bore and stroke of the pistons. Formula One engines can rev up to 15,000 rpm as per Formula One rules because of their smaller displacement, low mass, and short stroke.
Engines with hydraulic tappets (such as the Buick/Rover V8) often have in effect a rev limiter by virtue of their design. The tappet clearances are maintained by the flow of the engine's lubricating oil. At high engine speeds, the oil pressure rises to such an extent that the tappets 'pump up', causing valve float. This sharply reduces engine power, causing speed to drop.
The RPM level that results with the spark being arrested can be a constant level, or, with the proper ignition control modules, variable. Variable rate ignition modules can be adjusted quickly and easily to achieve the appropriate RPM limit for different situations, such as street racing, drag racing, road course racing and highway driving.
Multiple stage ignition modules offer greater RPM limit control. The first stage can be used to limit RPM levels when launching a vehicle from a stationary position, providing maximum power and traction. The second stage is activated after launch to set a higher RPM limit for wide-open-throttle acceleration.
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